Fixed bed coal gasifiers have been known and used for many years. Successful operation thereof has depended upon the use of non-caking coals or coke to accomplish the gasification. The instant invention is directed to an improvement in the coal gasification apparatus whereby the detrimental effects of using coals of high swelling and caking index are, at least in part, offset.
In the fixed bed coal gasification process, as the coal enters the gasifier at the top and lands on the top of the charge, it receives heat from the upwardly-moving product gas. As the combustion process near the bottom of the gasifier consumes some of the coal and the rest of the coal is gasified, coal at the top of the charge gradually moves down passing through a series of treatment states: initial heating; devolatilization and coking; gasification, and carbon oxidation. A large number of chemical reactions occur and a minimum temperature of about 1700.degree.-1900.degree. F is required. When air and steam are introduced to the combustion zone, the end product is producer gas; when oxygen and steam are employed, the end product is synthesis gas.
In the typical coal gasifier the feed is nitroduced into the gasifier by means of a charging lock arrangement and the ash is removed by means of a discharge lock. Each lock is provided with closure arrangements.
An improved coal gasification process in which finely divided coal with a binder is extruded into the coal gasifier with the gasifier being operated at elevated pressures is described in copending U.S. patent application Ser. No. 526,228 -- Furman, filed Nov. 22, 1974, now abandoned, and assigned to the instant assignee. The construction utilized in the latter invention considerably decreases the headroom required in the building housing the gas generator and eliminates the problem of disposing of the made gas entering the charging lock during coal feed via a charging lock arrangement.
A fuel-bed agitator or stirrer construction consisting of a vertically mounted rotating shaft to which are affixed three pairs of horizontally extending arms, or rabbles, vertically disposed two feet apart is disclosed in Bureau of Mines Report of Investigations 7644 "Strongly Caking Coal Gasified in a Stirred-Bed Producer", Lewis et al., U.S. Department of the Interior (1972). The agitator construction is shown in FIG. 2 on page 4 of the report. The Bureau of Mines report is incorporated by reference with respect to its disclosure of a gas producer equipped with a stirrer and means for affecting vertical and rotational motion thereof.
Deflection of the distal end of the shaft in such an arrangement can occur by the imposition of a relatively small (e.g., 100 pounds) of side thrust such that the lower rabble arms will be brought into contact with the wall and, if the wall were to be made of ceramic brick, a relatively soft material, the rabble arms would dig into and damage the brick wall. When the portion of the gasifier wall in which such damage could occur is made of water-cooled metal, as is shown in FIG. 3 of the Bureau of Mines report, damage is minimal.
However, in using a water-cooled wall, certain disadvantages accrue; namely, the coal adjacent the cooled wall is not properly gasified and may enter the ash zone before suitable decomposition thereof. In this condition in addition to decreasing the efficiency of the gasification, the coal promotes the development of large slag-like clinker deposits affixed to the walls that are difficult to break up in order to discharge the ash through the grate. Thus, it would be of advantage to be able to use a depending stirrer construction of the general configuration illustrated in the Bureau of Mines report, but have the gasifier wall made of ceramic. It is to the solution of this problem that the instant invention is addressed.